This invention relates to air turbine engine starters and more particularly to air turbine starter valves controlling the flow of compressed air to such starters.
Air turbine type starter motors are driven by the energy of a compressed gas such as air and are often used for starting an aircraft gas turbine, or jet, engine. Honeywell International, Inc., the developer of this invention, is a leader in the air turbine starter business. Compressed air flows to the starter which causes rotation of the compressors and the turbines within the jet engine. Upon sufficient air flow through the jet engine (reflected by turbine speed or otherwise), the jet fuel can be ignited within the combustion area/combustor to start the engine. Without the compressor/turbine rotation provided by the starter, fuel combustion and air flow through the engine will not be sufficient to start the engine. The compressed air for the air turbine starter is controlled by a starter valve, such as an air regulating and shutoff butterfly valve.
A source of relatively clean dry air is desired to power the air turbine starter. The most common sources of compressed/pressurized air for this purpose are an auxiliary power unit, bleed air from the compressor stage of another operating gas turbine engine, or a gas turbine ground power cart. Upon actuation of the engine start switch, the starter valve is energized and opens at a controlled rate to permit air to flow to the air turbine starter. The air turbine starter valve output air flow spins components of the air turbine starter motor, which converts the energy in the moving air to torque. This torque is applied to the engine gearbox which is then accelerated to a predetermined cut off speed whereupon the engine can start. The pilot may manually terminate this start cycle by opening the start switch. Automatic termination may be provided for by a speed sensitive switch built into the starter or by a main engine speed signal read by a fully-automated digital engine controller, commonly known as an FADEC. When the start cycle is terminated, the starter valve is closed, thereby cutting off the airflow that powers the air turbine starter. When the starting air flow is cut off, the air turbine starter automatically disengages from the engine accessory drive shaft and comes to a stop.
The starter valve controls the operation of the air turbine starter by controlling the rate at which it opens and closes and/or by a pressure regulating system that delivers substantially constant pressure to the starter regardless of the upstream air pressure. These functions in a conventional starter control valve may be implemented by mechanical-pneumatic control devices such as orifices, needle valves, springs or diaphragms. Limitations of these devices may include excessive design and manufacturing complexity, difficulty of adjustment, sensitivity to environmental changes and poor repeatability.
The starter control valve should control the pressure of air initially supplied to the air turbine starter to prevent destructive shock to the mechanism. As the starter speed increases, the rate of increase in air pressure is typically progressive to effect a smooth, rapid acceleration of the mechanism. In addition, the control valve may serve to maintain air pressure by responding to the air pressure sensed on the upstream side of the air turbine starter valve.
A control valve of this type should regulate pressure, limit pressure rise rate, and control the speed of the air turbine starter. It is also desired to meet specific speed requirements over a wide range of changing loads. Moreover, control valves usually do not provide high frequency response because of the difficulty in controlling valve dynamics and nonlinearities such as friction and aerodynamic forces.
One challenge that has arisen in the use and implementation of conventional starter control valves is the obstruction of the valve by ice. In particular, when an aircraft is on the ground, moisture present under cold and humid conditions can freeze the starter valve shut and thereby prevent initial engine start. When an aircraft is in the air, particular cold conditions of high altitude flight may cause ice to freeze the valve shut and prevent the restart of an engine after it shuts down. While certain remedies are currently available, such as providing a warming blanket or the like, the ice that forms is generally from only 2-5 milliliters of water. This amount of ice can generally be broken by the application of sufficient force to free the starter valve. Conventional starter valves, while sufficiently safe, may be, under certain circumstances, unable to provide such force to break the starter valve free of the ice.
One air turbine starter valve uses a force-offset actuator that produces a low torque at minimum pressure. While being sufficiently safe and reliable, this low torque can be insufficient to break an ice build up around the butterfly plate that controls air flow through the valve. Thus, a need exists for an air turbine starter valve that can function despite internal ice accumulation. The present invention satisfies this need.
The present invention provides an air turbine starter valve system that can break ice build-up that may form inside the valve under certain conditions. The actuator that serves to open the butterfly plate has two single-acting spring-return pistons connected to a common linkage. The dual actuators increase the torque available to open the butterfly plate by four times (400%). This significant increase in torque overcomes the frozen moisture that accumulates around the butterfly plate and breaks the ice to allow the valve to open after it has been frozen shut. Frozen valves opened by the dual actuator system or present invention, are able to open normally with minimum pressure. This eliminates flight delays, flight cancellations, or restarting problems at altitude.
Pressurized air flow approaches the butterfly plate and is sent to the actuators, which move through a displacement in accordance to the pressure applied. This displacement is supplied to a moment arm via common linkage between the two dual actuators. The torque then arising on the butterfly plate is sufficient to break frozen moisture about the valve, thus opening the plate and allowing the pressurized air to flow onward to the air turbine starter.